1. Field of the Invention
This invention relates to tungsten carbide button assemblies utilized in rotary rock cutters, and more particularly, to such a button assembly using significantly less tungsten carbide supported in a button which has a high abrasion resistance and a significantly higher transverse rupture strength than the tungsten carbide.
2. Background of the Prior Art
Rotary cutters, especially those utilized to penetrate subterranean rock strata, were originally utilized to penetrate relatively soft rock strata. Wear resistant tips of solid tungsten carbide were conventionally utilized on the cutter head of the rotary rock cutter and were fixedly secured thereto by means of silver solder. However, as those cutters were utilized to penetrate strata of increased hardness, the heat generated during such drilling melted the silver solder thereby causing the wear resistant tips to loosen and eventually to become unsecured. In addition, the silver solder bond would fail to hold the wear resistant tips when load pressures approximating 10,000 psi were exceeded.
It became imperative to develop rotary rock cutters which could withstand the increased load pressures needed to penetrate relatively harder subterranean strata. Rotary disc cutters were then developed to penetrate subterranean strata at load pressures of up to about 35,000 psi or greater where the strata is fractured. One category of conventional rotary disc cutters has a plurality of circumferentially extending discs usually constructed of steel which cut kerfs, or grooves, into the strata, and due to the wedging action thereof within the kerfs, create fracture planes in the strata. The fracture planes thus formed allow rock between the kerfs to break into relatively large chunks which may then be removed from the bore hole. In a second category, conventional hard rock rotary cutters have been developed to penetrate subterranean strata under load pressures of over 15,000 psi. Such hard rock cutters include both the tri-cone cutter and the large diameter big hole cutter which is employed in rotary drillings, such as applications of raise boring or tunnel boring. Hard rock cutters theoretically crush or break the rock strata penetrated thereby.
Both the steel disc cutters and the hard rock cutters employ wear resistant inserts or buttons in the cutting surface to extend the useful life thereof. Conventionally, an annular ring having a cross sectional, wedge shaped configuration is employed as the cutting surface for a rotary disc cutter. This annular ring is heat shrunk onto the rotary disc cutter and is formed of a wear resistant material, such as a 4330 or 4340 high alloy steel. Such annular rings are conventionally secured against relative movement with respect to the cutter by positioning each ring between a circumferentially extending annular shoulder and a plurality of pins which are suitably positioned around the circumference of the cutter. The selection of the specific material from which the annular ring is to be constructed is critical. If the material is too hard, the annular ring will fracture at high load pressures and if the material is too soft the annular ring will not withstand impact loads to which it is subjected and will deteriorate. These impact loads also tend to degrade the shrink fit causing the annular ring to loosen on the cutter. The soft material which the annular ring is constructed of cannot be heat treated to increase the hardness thereof since the material will become too brittle and the configuration of the bores into which wear resistant inserts are press fitted will change. Moreover, 4330 and 4340 high alloy steel which is commonly utilized to construct the annular ring is not relatively highly abrasion resistant, and therefore, rapidly erodes. Furthermore, it is not practical to fixedly secure the annular ring to the rotary disc cutter by means of silver solder. The large surface area of the ring cannot be sufficiently secured by soldering to withstand load pressures approximating 10,000 psi.
It is conventional to employ tungsten carbide button assemblies in the cutting surface of hard rock cutters since tungsten carbide is highly abrasion resistant, and therefore, extends the useful life of the rotary bit. However, conventional hard rock cutters utilize a relatively large number of buttons at relatively small spacings, e.g., one inch, to prevent high load pressures on any individual button. The height at which the buttons extend above the rotary bit surface is limited to about one-half inch since transverse shearing of the carbide inserts will occur when the insert is unsupported for over half an inch at these relatively high load pressures.
The relatively small distances the carbide inserts protrude from the rotary bit creates an additional problem of matrix washing. Crushed rock and rock chips may become positioned between the carbide buttons and contact the main matrix of the rotary bit thereby washing away or abrading the matrix. Extensive matrix washing can degrade the button support to the extent that the buttons loosen and eventually become unsecured from the rotary bit. Severe washing occurs because matrix material is machineable, and therefore, relatively low in abrasion resistance.
Several prior art patents relate to rotary rock cutters and wear resistant inserts or button assemblies therefore. U.S. Pat. No. 2,121,202 (issued on June 21, 1938 to Killgore) discloses tapered hard metal inserts for both rotary disc cutters and roller cutters of cylindrical and conical form. The tapered inserts are positioned in correspondingly tapered holes or openings in the cutter matrix, and held in place by the gripping effect of the matrix metal. The inserts may or may not be bottomed in the openings. Further, the inserts and openings of Kilgore may be untapered or cylindrical and the inserts may be heat shrunk into the matrix. As utilized in a disc cutter, the inserts provide peripherally positioned teeth. These teeth may be reinforced by the placement of hard metal bodies therebetween. When extremely abrasive formations are encountered, the breaking of the cutting teeth may be minimized by employing a composite insert positioned within the insert. This composite insert is constructed of a diamond metal, such as tungsten carbide, and the insert is constructed of a hard metal, such as one of the tough and hard, high speed, or air-hardened alloys. The tapering of the inserts provide the structural strength.
U.S. Pat. No. 3,311,181 (issued on Mar. 28, 1967 to Fowler) relates to rock drill teeth positioned in sockets formed in the drill bit matrix. Each tooth is split along its longitudinal axis thereby forming a working section and a correspondingly configured holding section. The working section is formed of a material having considerably greater hardness than that of the holding section or the bit matrix, such as, inter alia, tungsten carbide. The holding section and the bit matrix can be of any suitable material, preferably the matrix being of a somewhat softer material than the holding section. Thus, the material of the holding section erodes more rapidly than that of the working section thereby exposing effective portions of the working section for continued cutting action. The composite tooth may be cemented or heat shrunk in the sockets. Alternatively, the longitudinal split may be formed so as to provide a wedgeshaped holding section to additionally support the working section within the bit matrix.
U.S. Pat. No. 3,693,736 (issued on Sept. 26, 1972 to Gardner) discloses several configured inserts and cooperative sleeve assemblies for use in the solid and rotary type rock bits. The composite cutter insert assemblies comprise a tungsten carbide cutter element having a sleeve jacket secured therearound, such as by a press fit. The sleeve is preferably constructed of steel having a high yield point and high ductility so that the sleeve will wear away during drilling to continuously expose additional carbide until the cutter element is worn out. The sleeve may then be machined to allow replacement of the worn out carbide cutter element. The composite insert may be secured within the bit body by a press fit, by brazing or silver soldering techniques. In certain disclosed embodiments, the sleeve jacket may protrude beyond the bit body.
U.S. Pat. No. 3,749,190 (issued on July 31, 1973 to Shipman) relates to a sleeve having either a uniform wall thickness or a tapered wall which is inserted between a tapered carbide button positioned within a cylindrical bore in a rock drill bit matrix. The bore has an annular undercut near the lower end thereof. The sleeve is wedged between the carbide button and the bore wall with sufficient force to extrude part of the sleeve into the undercut so as to increase the retaining strength of the sleeve. The sleeve does not protrude from the bit body.
U.S. Pat. No. 3,771,612 (issued on Nov. 13, 1973 to Adcock) relates to replaceable wear resistant cutting elements for earth drilling, crushing and engaging equipment. The cutting elements have a wear resistant cutter insert or button surrounded (except at the forward end thereof) by a relatively tough, non-brittle hardened alloy steel sleeve which is preferably longitudinally split. Such longitudinal split allows the sleeve to be more resilient by permitting a large degree of circumferential expansion and contraction. The sleeve and cutter insert or button are mounted within a recess in the drill bit by means of an anvil stool or pedestal. This anvil stool or pedestal has a shearable means projecting radially outward from the stem thereof for supporting the sleeve during normal operation and for enabling the sleeve to be moved towards the back wall of the recess thereby releasing the wear resistant element or button for replacement. The sleeve normally projects slightly outward from the bit body.
U.S. Pat. No. 3,852,874 (issued on Dec. 10, 1974 to Pearson) discloses a method of inserting a button sleeve assembly into a slightly oversized bore in a rock drill head or the like. The bit and sleeve can be manufactured and stored as an integral unit and then adapted for installation in various size bores in rock drill heads. The sleeve extends outwardly beyond the bore thus protecting the edge of the bore from damage due to percussive loading on the button. Preferably the sleeve is made from 4340 high alloy steel and then heat treated to produce a hardness of 38 to 40 Rockwell so that it can be machined.
All of these prior art approaches share common problems in that the supporting sleeve (in the Fowler patent the holding section of the drill teeth) is not designed to penetrate or cut, and therefore, erodes away in a relatively quick manner. In fact, Pearson and Gardner are designed to be machinable or to wear away. The tungsten carbide insert or button in each prior art approach are designed to perform the cutting. Tungsten carbide which has long been utilized for its high abrasion resistance cutting qualities has a low transverse rupture strength causing fracturing and breaking. Thus, the tungsten carbide inserts or buttons are constructed to protrude only a very short distance from the bit body which, while aiding in minimizing shearing and fracturing of the insert or button, also reduces the useful cutting life thereof.
In addition, the prior art inserts or buttons are manufactured so as to have a relatively large portion of the tungsten carbide below the matrix surface and to have a large diameter with respect to the supporting sleeve in an attempt to minimize the fracturing problem and in order to hold the carbide in the matrix. As wear resistant materials, such as tungsten carbide utilized to construct inserts or buttons are relatively expensive, the cost of prior art approaches are relatively high. The cost of manufacturing rock cutters equipped with prior art insert or button assemblies is further increased by the relatively large number of inserts required to withstand high load pressures.
It can be appreciated that a need exists for abrasion resistant button assemblies for rotary rock cutters having extended useful cutting lives and reduced costs of manufacturing especially in the amount of tungsten carbide being used.
As will become apparent in the following, the button assembly of the present invention utilizes a button having high abrasion resistance and a significantly higher transverse rupture strength than the tungsten carbide which cooperates with the tungsten carbide insert to provide effective cutting at a significant cost reduction due to the small amount of tungsten carbide actually used and to provide significant longer wearing capability due to the lengthy extensive protrusion of the button of the present invention over prior approaches. Due to this extensive protrusion, compacting of rock around the button assembly is minimized and chip relief is maximized. Less energy is required to operate cutters using the button assemblies of the present invention due to the substantially increased chip relief. The increased chip relief also reduces matrix washing around the button assemblies of the present invention thereby prolonging the life of the cutter and minimizing loss of buttons due to washing. Furthermore, the button assemblies of the present invention are designed not to be replaced.
The cooperation of the button with the tungsten carbide insert results in an interaction that continually causes the tip of the carbide to impact the rock strata and the sides of the button to impact the rock while the button continually provides support to the insert to minimize transverse rupturing of the carbide. Because of this cooperation, the button assembly of the present invention is capable of providing the first practical application of tungsten carbide to rotary disc cutters.
In addition to the above prior art references, which were uncovered in a patentability search, the following references were also uncovered but were not considered to be as pertinent: Kreag, U.S. Pat. No. 2,065,898, "Tool and Method of Making Same" (Dec. 29, 1936); Killgore, U.S. Pat. No. 2,161,062, "Percussion Tool" (June 6, 1939); and Kniff, U.S. Pat. No. 3,807,804, "Impacting Tool With Tungsten Carbide Insert Tip", (Apr. 30, 1974).